Selectivity control for radio



March 10, 1933. F U T 2;o33,330

SELECTIVI'I'Y CONTROL FOR RADIO Filed Sept. 21, "1934 5 Sheets-Sheet l Y INVENTOR.

[54/5 [KMFT/s BY ivkfluwdu A TTORNEY.

March 10,1936. F, -n5 2,033,330

SELECTIVITY CONTROL FOR RADIO Filed Sept. 21, 1934 3 Sheets-Sheet 2 *eax *sax

A146. ems

FELA) 70a:

INVENTOR.

'A TTORNEY.

Patented Mar. 10, 1936 I I v UNITED STATES PATENT OFFICE SELECTIVITY CONTROL FOR RADIO Leslie F. Curtis, Springfield, Mass, asslgnor to United American Bosch Corporation, Springfield, Mass., a corporation of New York Application September 21, 1934, Serial No. 744,892 E S S U D 18 Claims. (01. 250-20) This invention relates to radio receivers and ployed in the intermediate amplifier. Fig. is related equipment a d m e p ticu ly to a diagrammatic representation of an alternative superheterodyne receivers in which the overall form of the'invention in which a positive feedselectivity may be altered to suit changing reback is applied to an intermediate frequency 5 quirements. The intermediate frequency ampliamplifier.

fler of a superheterodyne usually consists of a Figs. 1 and 5 show systems of selectivity connumber of circuits tuned to a particular fretrol using positive feedback in which inherently quency and the response curve of the amplifier broad tuned amplifiers may be made to tune has its highest point at that frequency. For more sharply as requirements change. adjacent frequencies the response is not so great Referring to Fig. l in more detail, the numand the curve slopes off on either side. Such a eral I represents the primary of an intermediate curve would indicate a selective receiver, but one frequency transformer having a secondary 2. in which tonal quality was sacrificed for selec- A variable condenser ,3 tunes the secondary tivity. This condition is known as -side-band circuit to the desired intermediate frequency of cutting and results in a thin, colorless audio the amplifier. The control grid of an amplifier 5 reproduction. Commercial practice has been to tube 4 receives the output of the tuned secondcompromise these conflicting requirements, and ary circuit. The tube l may be of the type to produce a receiver in which neither the seleccommercially designated as "58 and is contivity nor the tone is all that may be desired. n'ecte'd in a well known manner to function as The intermediate amplifier was adjusted and an amplifier, the particular connections form- 20 remained fixed regardless of insuflicient selecing no part of this invention. The ampl fi tivity to separate a weak. distant signal from output of tube 4 passes to a tuned primary ciradjoining signals, and regardless of the full Cu t comprising t e O m P a y 5, tonal possibilities available in a strong local tu ed by t V b condenser The transsignal. former secondary I is likewise tuned by the vari- 25 The object of this invention is to provide a able Condenser A Small condenser 9 is 1 radio receiver in which the overall selectivity nected between the ungrounded side of secondmay be varied in accordance with the incoming y Circuit and the cathode of tube signal strength. The cathode current of tube 4 is returned to Another object is to provide radio or related ground through resistance 0, and the volta 30 apparatus in which the overall selectivity i drop across resistance It] provides the necessary automatically controlled in accordance with the minimum bias for the grid of tube 4. The voltincoming signal strength. age developed across the secondary I8 is ap- A further object is to provide radio apparatus proximately out of Phase t the input in which an amplifier is adjustable with respect Voltage upp y the circuit 35 to selectivity by employing either positive or denser 9 the resistance in form a Phase negative feedback. correction and amplitude adjusting circuit which Still a further object is to provide a radio applies between the cathode of tube 4 and receiver in which the overall selectivity is conground 8 ac Voltage 111 ct P ase 40 trolled through the action of an a t ti which effectively increases the signal voltage be- 40 volume control. tween grid and cathode. A supplemental phase Other ob ects and advantages will in part b correction circuit such as is shown and ex-' stated and in part be obvious when the followplained in connection with Fig. 5 may be eming specification is read in connection with the pl d; if n ces y f r c rr ing m r pha accompanying drawings, in which: differences due to stray capacities and such.

Fig. 1 is a diagrammaticrepresentation of the The a ce l l is the Usual filter resistor preferred form of the invention in which a posip y d n autvma c Vo ume control circuits.. tive feedback is applied to an intermediate fre-" ypass Condense I2 is connected to the low quency amplifier; Fig. 2 is a. diagram of an potential side of the secondary circuit 23.

intermediate amplifier in which negative feeds- 2 Shows an alternative System61 c- 50 back is used to control the selectivity; Fig. 3 tivity trol in whic a sha ply u ed-amplifier is a block diagram of a receiver designed to em:- y b broadened for reception of Strong S g.- ploy a positive feedback in the intermediate nals- The components of the a fi pr per amplifier; and Fig. 4 is a block diagram of a, perform the same functions as in the showing receiver in which a negative feedback is em- 0f 8 1 and a d e pa ts in gs. 1, 2

and 5 bear the same reference characters. The voltage across the resistance 25 is fed through condenser 21 which forms a portion of a phase correction circuit including resistance 26. From condenser 27 the voltage goes through a coubias is applied to the control grid of a relay,

tube IS. The plate of tube It is connected through a resistance II to a 90 -volt source which is also connected to the cathode of the feedback amplifier tube IS. The tubes l5 and I6 have a minimum bias obtained by the drop across resistances l8 and I9 respectively. A lead including a filter resistance 20 connects the control grid of tube I! with the plate end of resistance H. The plate current of tube it flows across resistance ii and the bias on the grid of tube [5 is determined by the resulting voltage drop. When the automatic volume control bias is lowest. the tube It has a large plate current and the resulting voltage drop across resistance l'l places a high negative bias on the gridof tube l5, which then passes substantially no feedback voltage. Conversely, when the-automatic volume control bias is greatest, tube l6 has very little plate current. The voltage drop across resistance I1 is then practically zero, the added negative bias is removed from tube l5, and there exists maximumv amplification of the feedbackvoltage. Any intermediate automatic volume control bias produces a simultaneous change in the value of the amplified feedback voltage. The output of tube It which constitutes the amplifier feedback voltage isconnected to a potentiometer 2 I. A bypass condenser 22 is connected between the supply end of potentiometer 2| andground. The slider arm of potentiometer 2| takes on! any desired portion of the amplified feedback voltage and it is fed through a coupling condenser 23 to the control grid of tube 4. Two feedback amplifier tubes in cascade may be employed in case a greater amplification is desired. The automatic volume control system controls the tube 4 in the usual manner through the lead including the filter resistor 24.

Figs. 3 and 4 are block diagrams of the systern which show a positive and a negative feedback respectively in radio receivers.

Fig. 5 shows 'a' modification of the-positive feedback system shown in Fig. 1. A resistance 25 is serially included in the circuit I8, and

' has one end grounded. The voltage appearing across resistance 25 is taken off the ungrounded end of the resistance. This voltage will be approximately 90 ahead of the phase of the voltage appearing across the secondary winding I, and is led to an auxiliary phase correction A blocking condenser 28 of a value large enough to readily pass the modulated intermediate frequency is connected to the phase shifting circuit,and the control grid of tube 4 is connected compatible.

amplifier coils.

justment.

to the other side of condenser 28. The automatic volume control bias is fed through a resistance ii of large value which is connected between the control grid of tube 4 and condenser 28. The voltage appearing across the secondary I is amplified by the tube I4 and may be passed to asucceeding stage, not shown High fidelity and extreme selectivity are not Side band cutting due to extreme selectivity may be reduced by over-coupling the tuned circuits, thereby obtaining a double peaked? or flat topped" selectivity curve which is perfectly satisfactory when receiving strong signals. For weak distant stations a much greater selectivity is needed to avoid adjacent channel interference from stronger signals.

While this interference may be reduced by em-.

playing a tone control to dampen the higher audiofrequencies, the output remains distorted to a large extent. Another remedy has been to vary the selectivity of a receiver through mechanically changing the degree of coupling between tuned circuits by ganging adjustable elements to control the spacing of intermediate This last method is mechanically cumbersome and prone to get out of ad- In the present invention a single manual or automatic adjustment of feedback in an amplifier may be used to control the side band cutting and so control the audio fidelity of a radio receiver. The effect is similar to that obtained by manually adjusting the. coupling between intermediate circuits, but without the mechanical complications of this method.

High fidelity response requires that the reproduction from 50 to 7500 cycles per second be uniform within a range of a few decibels. The feedback principle accomplishes this for strong signals, but allows the receiver to function with maximum selectivity on weak signals.

If the feedback voltage were introduced directly from the output to the input of a stage, there would be a phase difference of approximately 90 from the proper value. The effect of resistance 25 in series with condenserfl forms a phase changing circuit as shown in Figs. 2 and 5, which provides the approximately correct phase of feedback, and remains approximately constant over the range of frequencies comprising the side bands of the modulated signal. In actual practice, the effective capacitance of resistors and other units, loses in the coils and shielding, which are not included in the original calculations, may cause a slight additional phase shift, clockwise or counter-clockwise. This may be corrected by a proper selection of resistance 26 and capacitance 21 in the phase correction circuit. Correction for an opposite phase may be made by reversing the relative position of these elements. i

For a given selectivity curvesthemathematical product of basic amplification and-feedback must remain constant. In an intermediate frequency amplifier having. an automatic volume control, the basic amplification without feedback varies with the grid bias of the tubes. For week signals the greatest selectivity is desired, while strong signals require less selectivity and more fidelity intransmission. This may be accomplished manually by an attenuator in the feedback circuit; however the preferred method is to automatically adjust the feedback to the correct value in proportion to received signal strength as shown in- Figs. 1 and 2.

Positive feedback The operation of the positive feedback circuit which is shown in Fig. 1 is as follows: The desired feedback voltage in correct amplitude and phase is obtained across resistance N by serially connecting the resistance Ill and condenser 8 between ground and the high potential side of the secondary circuit |--8. This voltage is ap-' plied to the cathode of tube 4. 'The resistance I I is a filter resistance, serving to keep high frequency voltages out of the balance of the automatic volume control system. The positive feedback is used to make the receiver more selective by symmetrical regeneration. A receiver employing this principle is originally constructed to have a broad selectivity curve, and to amplify the audio side bands of the signal in approximately an equal amount. The receiver will then reproduce with excellent fidelity, but will originally be lacking in selectivity. I For a weak signal input, where selectivity is required, the positive feedback in correct phase peaks the selectivity curve by symmetrical regeneration. This peaking effect'of a given feedback is greatest when the basic amplification is greatest. In an automatic volume control receiver the amplification is greatest when the signal is'weak and thus the feedback is most effective in producing a sharp selectivity curve. Upon the occurrence of a strong signal, the amplification is low, the feedback has little effect on the selectivity, and l the curve shape returns to approximately its normal form for the tuned circuit without a feedback. While the feedback may extend over more than one stage of the amplifier, it is then difficult to reinforce the 7000 cycle audio side band when an intermediate frequency of 456 kilocycles is used. Increasing the intermediate frequency sufiiciently to accomplish this would probably place it in the broadcast band, and so produce other difficulties. The preferred method of accomplishing the desired result is to provide an individual feedback circuit for each stage as shown in Fig. 3. 1

Negative feedback A receiver employing negative feedback to control selectivity, as is shown in Figs. 2 and 4, is originally constructed to have. a sharp selectivity curve. Selectivityv requisite to separating a weak signal from an adjacent stronger one is originally provided and when a strong station is received and tone becomes the paramount consideration, the selectiv ity curve is flattened sufiiciently to insure substantially equal passage of the audio side bands of the modulated signal. This requires a-wide range in the value of the feedback, and a feedback amplifier is provided to allow the desired. range. A strong signal requires a heavy negative feedback to sufilcie'ntly depress the amplifier selectivity curve, while for a weak signal the feedback should be removed in order to provide maximum selectivity and amplification. As negative feedback of correct phase isincreased the amplifier selectivity curve will flatten and then assume the double hump" characteristic of over-coupled circuits. This may be combined in the usual manner with the characteristic, of preceding circuits to produce a fiat-topped curve which is wide enough to avoid side band cutting. In this type of receiver, the automatic volume control bias determines the amount of negative feedback at a given time. In the positive feedback system,. the

ratio of feedback to signal is constant, and the automatic volume control bias regulates the amplification, which in turn determines the degree of effectiveness of the feedback iii-altering the selectivity of the amplifier.

It will be apparent that many changes and modifications may be made in .this invention by anyone skilled in the art and without departing from the true spirit and scope of the invention as defined in the following claims.

What I claim is:

1. In a radio receiver having an' amplifier, a

feedbackcircuit in said amplifier; a relay tube connected to said circuit and controlling said feedback, and means for controlling said relay tube in accordance with signal strength, whereby the selectivity of said amplifier is altered.

2. In a radio receiver having an amplifier, a

feedback circuit in said amplifier, phase Chang-- 20 ing means in said circuit, a' relay tube connected tosaid circuit and controlling said feedback,

and means for controlling said relay'tube in accordance with signal strength, whereby the selectivity of said amplifier is altered.

3. In a radio receiver, an amplifier having predetermined band-pass characteristics, an electron tube in said amplifier having an input and an output circuit, an energy feedback path from said output to said input circuit, and means including the feedback of energy over said path, whereby the band-pass characteristics of said amplifier may be altered in accordance with the intensity level of substantially any signal capable of affecting said receiver.

4. In a radio receiver, an amplifier having a predetermined selectivity characteristic, an electron tube in said amplifier having an input and an output circuit, an energy feedback path from said output to said input circuit, and means including the feedback of energy over said path, whereby the selectivity of said amplifier is varied inversely with the strengthof substantially any signal capable of affecting said receiver.

5. In a radio receiver, an electron tube amplifier havinga predetermined selectivity characteristic, an automatic volume control regulating the amplification of said amplifier, said control being operable over substantially the entire range of signal intensities capable of affecting said receiver, a feedback circuit in said amplifier, and a;tube'in said amplifier receiving .both feedback and automatic volume control potential, whereby the selectivity characteristic of said amplifier varies with variations in the amplification thereof.

'6. In a radio receiver, an electrontube amplifier having predetermined band-pass characteristics, a. volume, control in said receiver, and means for imposing a negative feedback potential upon a tube in said amplifier, said means acting jointly with said volume control for altering the band-pass characteristics of said amplifier. g

7. In a radio receiver, an electron tube amplifier having predetermined band-pass characteristics, an automatic volume control system, and means for imposing a negative feedback potential upon a tube'in said-v amplifier, said potential varying with different'signal intensities, said means acting jointly with said volume control in altering the band-pass characteristics of said amplifier.

8. In a radio receiver, an electron tube amplifier having predetermined band-pass character istlcs, an automatic volume control system, and

-means for imposing a negative feedback potential upon a tube in said amplifier, said potential varying with dlflerent signal intensities, said means being regulated by said automatic volume control, whereby the band-pass characteristics I of said amplifier are altered.

I said circuit for imposing. a predetermined phase condition upon the energy in said circuit, said volume control and said energy feedback acting Jointly to alter the selectivity characteristic of said amplifier. I

10. In a radio receiver, an amplifier having a predetermined selectivity characteristic, an automatic volume control in said receiver, a nega-- tive feedback circuit in said amplifier, and a phase adiusting means in said circuit for imposing a predetermined phase condition upon the energy in said circuit, said automatic volume 1 control regulating the amount of negative feedback in response to received signal strength whereby the selectivity characteristic of said amplifier is altered. c

11. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a volume control regulating the amplification of said amplifier, and means including the feedback of energy over said path acting in response to volume control action, whereby the selectivity of said amplifier may be .varied in response to substantially all received signal'intensities capable of affecting said receiver.

12. In a radio receiver, an electron tube ampli fier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an ene y feedback path from said output to said input circuit, means for varying the mutual conductance .of said amplifier, and means including the feedback of energy over said path acting in response to variations in mutual conductance, whereby the basic selectivity of said amplifier may be varied in response to substantially all received signal intensities capable of affecting said receiver.

13. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a variable grid bias volume control system connected to said amplifier, said control being operative over path acting in response to variations of grid bias to alter the selectivity of said amplifier.

14. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit cohnected to said amplifier, means for producing a =feedback of energy from said output to said input circuit, a volume control regulating the output of said receiver, said means acting jointly with said volume control to narrow the selectivity characteristic of said amplifier simultaneously with reductions in received signal intensity over substantially the entire range of said intensities to which said receiver is responsive.

15. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a volumecontrol operable over substantially the entire range of received signal intensities capable of afiecting said receiver, means for imposing a positive feedback of energy over said path, said feedback energy acting jointly with said volume control to alter the selectivity of said amplifier.

16. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and'an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a variable grid bias volume control operable over substantially the entire range of received signal intensities capable of affecting said receiver, means for imposing a positive feedback of energy over said path, said feedback energy acting jointly with said volume control to alter the selectivity of said amplifier.

17. In a radio receiver, an electron tube amplifier having a predetermined selectivity, characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a variable rid bias volume control operable over substantially the entire range of received signal intensities capable of ailecting said receiver, means for imposing a constant percentage of feedback energy over said path, said feedback energy acting Jointly with said volume control to alter the selectivity of said amplifier,

18. In a radio receiverhaving an amplifier, a feedback circuit in said amplifier, a relay tube connected to said circuit and controlling said feedback, an automatic volume control system in said receiver, and a connection between said systern and said relay tube whereby feedback is regulated by the control potentials "developed by said systems and the amplifier selectively varied substantially the entire range of received signal accordingly.

intensities 'capable'of aifecting said receiver, and LESLIE F. CURTIS,

means including a feedback of energy over said DISCLAI MER' 2,033,330.Lesl'ie F. Curtis, Springfield, Mass. SnLnc'nvrrY CoN'rRoL FOR RADIO.

Patent dated March 10, 1936. Disclaimer filed March 8, 1938, by the patentee; the assignee, Hazelt'ine Corporation, assenting.

Herelg enters this disclaimer to claim fl'ic'ial Gazette March 29, 1938.]

1 of said patent. m

-means for imposing a negative feedback potential upon a tube in said amplifier, said potential varying with dlflerent signal intensities, said means being regulated by said automatic volume control, whereby the band-pass characteristics I of said amplifier are altered.

I said circuit for imposing. a predetermined phase condition upon the energy in said circuit, said volume control and said energy feedback acting Jointly to alter the selectivity characteristic of said amplifier. I

10. In a radio receiver, an amplifier having a predetermined selectivity characteristic, an automatic volume control in said receiver, a nega-- tive feedback circuit in said amplifier, and a phase adiusting means in said circuit for imposing a predetermined phase condition upon the energy in said circuit, said automatic volume 1 control regulating the amount of negative feedback in response to received signal strength whereby the selectivity characteristic of said amplifier is altered. c

11. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a volume control regulating the amplification of said amplifier, and means including the feedback of energy over said path acting in response to volume control action, whereby the selectivity of said amplifier may be .varied in response to substantially all received signal'intensities capable of affecting said receiver.

12. In a radio receiver, an electron tube ampli fier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an ene y feedback path from said output to said input circuit, means for varying the mutual conductance .of said amplifier, and means including the feedback of energy over said path acting in response to variations in mutual conductance, whereby the basic selectivity of said amplifier may be varied in response to substantially all received signal intensities capable of affecting said receiver.

13. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a variable grid bias volume control system connected to said amplifier, said control being operative over path acting in response to variations of grid bias to alter the selectivity of said amplifier.

14. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit cohnected to said amplifier, means for producing a =feedback of energy from said output to said input circuit, a volume control regulating the output of said receiver, said means acting jointly with said volume control to narrow the selectivity characteristic of said amplifier simultaneously with reductions in received signal intensity over substantially the entire range of said intensities to which said receiver is responsive.

15. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a volumecontrol operable over substantially the entire range of received signal intensities capable of afiecting said receiver, means for imposing a positive feedback of energy over said path, said feedback energy acting jointly with said volume control to alter the selectivity of said amplifier.

16. In a radio receiver, an electron tube amplifier having a predetermined selectivity characteristic, an input and'an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a variable grid bias volume control operable over substantially the entire range of received signal intensities capable of affecting said receiver, means for imposing a positive feedback of energy over said path, said feedback energy acting jointly with said volume control to alter the selectivity of said amplifier.

17. In a radio receiver, an electron tube amplifier having a predetermined selectivity, characteristic, an input and an output circuit connected to said amplifier, an energy feedback path from said output to said input circuit, a variable rid bias volume control operable over substantially the entire range of received signal intensities capable of ailecting said receiver, means for imposing a constant percentage of feedback energy over said path, said feedback energy acting Jointly with said volume control to alter the selectivity of said amplifier,

18. In a radio receiverhaving an amplifier, a feedback circuit in said amplifier, a relay tube connected to said circuit and controlling said feedback, an automatic volume control system in said receiver, and a connection between said systern and said relay tube whereby feedback is regulated by the control potentials "developed by said systems and the amplifier selectively varied substantially the entire range of received signal accordingly.

intensities 'capable'of aifecting said receiver, and LESLIE F. CURTIS,

means including a feedback of energy over said DISCLAI MER' 2,033,330.Lesl'ie F. Curtis, Springfield, Mass. SnLnc'nvrrY CoN'rRoL FOR RADIO.

Patent dated March 10, 1936. Disclaimer filed March 8, 1938, by the patentee; the assignee, Hazelt'ine Corporation, assenting.

Herelg enters this disclaimer to claim fl'ic'ial Gazette March 29, 1938.]

1 of said patent. m 

